Conventionally, for a water quality contamination monitoring system for measuring dissolved oxygen in water, water pollution, water quality, etc. with a sensor or the like, a detecting place is at a distance from a monitoring station, moreover there is no power supply to supply electric power to the sensor at the detecting place in some cases.
As described above, in the case which there is no power supply at the detecting place, a system for supplying electric power to a sensor by transmitting light from a monitoring station to a detecting place through an optical fiber, and then converting the light to electric power can be employed according to Patent Reference 1.
That system comprises a sensor unit 110 positioned at the detecting place and a measurement device 120 positioned at the monitoring station as illustrated in FIG. 15.
In the sensor unit 110, a supply circuit 112 for supplying electric power to a sensor 111, a light/electric power converter 113 for supplying electric energy to the supply circuit 112, and a LED 114 for receiving output signals from the sensor 111 are provided. The LED 114 and the light/electric power converter 113 are adjacently provided.
An optical fiber 115 is led from the measurement device 120 into the sensor unit 110, and the optical fiber 115 is a single core type and has functions for receiving signal light emitted from the LED 114 as well as irradiating light energy to the light/electric power converter 113.
In the measurement device 120, a light input/output device 121 connected to another end of the optical fiber 115, a micro computer 122 connected to the light input/output device 121, and a battery 123 for supplying electric power to the micro computer 122 are provided. The light input/output device 121 incorporates a light source (not shown) for irradiating light to the light/electric power converter 113 and a light receiving element (not shown) for receiving the optical signal transmitted via an optical fiber 115. The signal electrically converted by the light receiving element is processed by the micro computer 122.
However, in such system, one piece of optical fiber 115 must be arranged in the sensor unit 110 so that the light input/output ranges of the light/electric power converter 113 and the LED 114 are covered. As a result, the distance from an end surface of the optical fiber 115 to the light/electric power converter 113 and the LED 114 becomes longer. In addition, since a part of light irradiated to the light/electric power converter 113 is injected into the LED 114, the electric energy which can be supplied to the sensor 111 becomes smaller.
The less electric power supplied to the sensor 111 is, the weaker optical output signals from the LED 114 is, accordingly not only the optical fiber lead-out distance is limited, but also detection accuracy tends to degrade with taking into account of optical attenuation. Therefore, such systems are used in a car, or the like where a sensor unit and a measurement unit are arranged very close to each other.
Patent Reference 1: Japanese Published Unexamined Patent Application No. H7-151563